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An attribute like <code>{attribute}</code> should be defined.
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<p>If it is not defined, AsciiDoc will usually print it as is in the output.
Having an undefined attribute in the source might indicate a mis-spelled attribute.
<p>To print a text in curly brackets as is, mark it as pass through content like <code>+{attribute}+</code>
<p>An attribute will be treated as defined if it is declared in at least one of the following places:
<ul>
    <li>Any AsciiDoc document within the project for non-Antora setups, or within any Antora page in any module of the project for Antora setups.</li>
    <li>Any <code>.asciidoctorconfig</code> or <code>.asciidoctorconfig.adoc</code> file in the current project.</li>
    <li>The Antora module descriptor of the current module.</li>
    <li>Any Antora playbook within the current project.</li>
    <li>Attributes set in the IDE's AsciiDoc settings.</li>
    <li>A well-known AsciiDoc attribute like for example <code>{table-caption}</code> or <code>{docname}</code>.</li>
</ul>
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